Structural controls on mineralized veins in the Sweet Home mine, Alma district, Colorado

The Sweet Home mine is located in the Mosquito Range of central Colorado,
where normal faults of the Rio Grande rift system are superimposed on the east flank
of a Laramide basement-cored anticline and the NE-trending Colorado mineral belt.
The Tertiary intermediate-sulfidation veins in the Sweet Home mine were originally
mined for silver (1873–1966), but from 1991 through 2004 the mine produced worldclass
rhodochrosite specimens. The veins are hosted in Precambrian granite gneiss
and formed in two stages at approximately 26 Ma as indicated by three new 40Ar/39Ar
ages. The first stage veins contain primarily quartz, sericite, and pyrite, with green
fluorite and hübnerite. The second stage veins contain silver and base metal minerals,
purple to blue fluorite, and rhodochrosite.
Structural mapping and analyses were performed to model structural controls of
the veins and their relationship to regional tectonics and the Colorado mineral belt.
Veins and faults were mapped underground and in surface exposures. The host rock
foliation strikes WNW, and dips 50° SW. All veins, faults, and fractures dip steeply
between 70° and 80°. First-stage veins and unmineralized faults strike NE, N, and
WNW, and second-stage veins strike dominantly NE. Slickenlines on vein surfaces
rake between 40° and 90°, although most rake steeply. It has been determined
through the use of Riedel steps that early stage veins show mixed normal and reverse
slip (mostly reverse slip) whereas second stage rhodochrosite veins show normal slip.
Dike offsets and oblique slickenlines indicate both dextral and sinistral slip components.
Stress modeling shows a near vertical maximum compressive stress (81°/090°)
and a northwest-trending extension direction for rhodochrosite veins. The Colorado
mineral belt in the northeast Front Range shows Paleocene (Laramide) dextral shear
during vein formation, but our data show post-Laramide Oligocene movement during
early Rio Grande rift extension.